This invention relates to novel metal coatings, which exhibit exceptional hardness. More particularly this invention relates to metal coatings containing nickel, boron and thallium to the reductive deposition of said coatings on the surfaces of substrate articles from aqueous solutions at an alkaline pH.
The plating or deposition of metal alloys by chemical or electrochemical reduction of metal ions on the surface of an article to modify its surface characteristics for both decorative and functional purposes is well known in the art. Of particular commercial significance is the deposition of metal/metal alloy coatings on both metal and activated non-metal substrates to enhance surface hardness and resistance to corrosion and wear. Nickel-boron and cobalt-boron alloy coatings are recognized in the art for their hardness and associated wear-resistance. The patent literature reflects an ongoing research and development effort in the area of nickel-boron coatings with the goal of producing still harder, more corrosion resistance coatings from a stable bath. For example, see, U.S. Pat. Nos. 5,019,163; 3,738,849; 3,674,447; 3,432,338; 3,378,400; 3,045,334; and 2,726,170. The art has recognized that when a borohydride reducing agent is used in a nickel/boron-plating bath a harder coating is achieved. However, borohydride, is very unstable in the bath. The solution to the stability problem has been to add stabilizers such as thallium salts such as thallium sulfate, or lead chloride to control the instability of the borohydride. The addition of stabilizers requires balancing the need for a proper plating rate with the need to control the stability of the borohydride. An over stabilized bath tends to plate slowly and co-deposit thallium in the coating.
The addition of the stabilizers created a new problem in the art by interfering with the formation of the nickel coating. During the formation of the nickel coating the stabilizer would co-deposit in the nickel coating thereby negatively impacting the hardness of the coating. In the case of thallium the hardness of the coating begins to decrease as the concentration of the thallium goes over three percent in the nickel coating.
As the bath ages there is a need to continuously add even additional thallium to achieve stability of the nickel/boron bath. During normal operation of the bath, boron and thallium salts are added every thirty minutes. As the bath ages the amount of thallium needed to stabilize the boroydride increases. This increase in concentration of thallium in the bath causes the concentration of the thallium in the nickel coating to increase. In a typical prior art bath such as Bellis or Klien the amount of thallium in the nickel coating can vary by as much as 50% over a two hour period of production. As the thallium concentration in the coating approaches 4% the hardness of the coating will be reduced by approximately 25%.
This invention solves the problem in the art of controlling the amount of thallium codeposited in the nickel coating as the bath ages while at the same time allowing for an acceptable plating rate. The inventor has discovered that by selecting a mixture of thallium sulfate and thallium nitrate the co-deposition of thallium in the nickel coating can be less than 4% as the bath ages. Preferably, thallium in the nickel coating can be less than 3% as the bath ages. At the same time the plating rate can be maintained at 1 mill per hour.
It is therefore a general object of this invention is to provide a method of electroless plating an article of manufacture or at least a portion of its surface with a hard, ductile, wear and corrosion resistant metal coating comprising both nickel, boron, and thallium from a bath containing a mixture of thallium sulfate and thallium nitrate so that the thallium codeposited in the coating is below 4%. Preferably the thallium codeposited in the coating is below 3%. And at the same time the plating rate can be maintained at 1 mill per hour.
An object of this invention is to provide improved metal alloy coating composition containing both nickel and boron and a mixture of thallium sulfate and thallium nitrate.
Another object of this invention is to provide coating baths from which a hard, ductile, wear and corrosion resistant coating can be deposited on at least a portion of the surface of a metal or activated non-metal substrate.